1. Field of the Invention
This invention relates in general to a receiver for optical digital signals having different amplitudes.
2. Description of the Prior Art
Receivers for optical digital signals usually contain a transimpedance amplifier which is coupled to the photodiode and also contains a comparator which is connected to the output of the transimpedance amplifier which emits voltage pulses according to the received light pulses. For example, the comparator output has a logical "1" when the photodiode receives light energy or, alternatively, when the photodiode does not receive light energy.
The optical signals received by such receivers usually arrive with power levels of different amplitudes since, for example, they have travelled light waveguide transmission paths which differ in length and/or have traversed light waveguides having different attenuation characteristics. The radiant intensity of the transmission diodes respectively employed can also vary from each other. In any case, it is desired that such receiver process optical signals which vary greatly from one another in power level without causing overdriving of circuits.
European Patent Application No. 0 052 221 which corresponds to U.S. Pat. No. 4,403,159 discloses a transimpedance amplifier that is realized with an operational amplifier. Using optical digital signals having a high power level, the inputs of the comparator following the operational amplifier are overdriven. Such overdriving results in delayed switching to a different signal status at the output of the comparator. Thus disturbing pulse distortion of the output signal occurs.
It is possible to couple the output of the transimpedance amplifier to the input of the comparator by way of a capacitor. Although pulse distortions can be in fact kept small in this manner, non-ambiguous relationship between the input signal and the output signal of the receiver is not established due to the absence of DC coupling.
Complicated receivers having automatic gain control are also possible. For the gain control to fulfill its object, the transmitter must be quasi-continuous, in other words, either demands must be made of the pulse duty factor or the transmitter must emit other auxiliary signals which the receiver can utilize for level control. A further disadvantage frequently results in that the control does not respond quickly enough when for example, data are sent from various transmitters to a receiver "packet by packet".
A transimpedance amplifier which has three amplifiers stages each having a transistor and with the first transistor operated in collector base circuit and the second is operated in emitter base circuit and wherein the transistor of the third amplifier stage has a load impedance at least at the emitter side and a feedback resistor is provided between the base terminal of the amplifier at the input and a circuit point of the amplifier that carries a signal in opposite phase to the amplifier input signal is disclosed by "Transimpedance Optical Pre-amplifier Having a Common Collector Front End" printed in Electronics Letters Vol. 18, No. 23, Nov. 11, 1982.